This invention relates to signal processing and more particularly to a method and apparatus for enabling the transition of an audio data signal converter between the active mode and the inactive mode, when link impairments, either transitory or sustained, exist in the data communication channel. This invention finds applications in digital communication systems, such as a digital cellular system or a Voice-over-IP (VoIP) system, in particular vocoder bypass capable systems that can selectively enable the activation or de-activation of the decoding and encoding functions in the connection.
In a digital communication system such as a wireless system or a VoIP system, an audio signal may be processed by a series of speech encoders and decoders as it is transmitted from one endpoint to another. In the example of a digital cellular mobile-to-mobile connection, the audio data signal is first encoded by a speech encoder at the first mobile telephone and transmitted in an encoded format to a base transceiver station of a cell site where it is transferred to the base station controller servicing that cell site. At the base station controller, the encoded speech information is processed by a compatible speech decoder that converts the compressed speech stream into PCM samples. The PCM samples are then transported over the landline network, such as the PSTN, toward the base station controller servicing the cell site communicating with the other mobile telephone. At the second base station controller, the PCM speech samples are again processed by a speech encoder. The encoded information is sent from the base transceiver station of the cell site to the second mobile telephone where the compressed speech stream is converted one more time by a speech decoder into PCM samples that can be used to generate an audio signal.
In this, codecs on both sides of the mobile-to-mobile call are connected in tandem, which is known to degrade the speech quality as a result of the successive encoding/decoding of the audio data signal.
The xe2x80x9cvocoder bypassxe2x80x9d technique alleviates this problem. During a connection between two base station controllers having identical codecs, the codecs are switched off when they are made aware of their mutual existence. Thus, the encoded speech information arriving at the first base station controller flows in encoded format through the PSTN and arrives as such at the second base station controller. Therefore, when in bypass mode, the data communication channel between the base station controllers transmits compressed data as opposed to speech samples. This procedure eliminates one decoding operation of the speech signal at the first base station controller and one re-encoding operation of the signal at the second base station controller. As a result, the audio quality is significantly improved.
For additional information on the xe2x80x9cvocoder bypassxe2x80x9d technique, the reader is invited to refer to the U.S. Pat. No. 5,768,308 granted to the present assignee that describes the process in great detail. The contents of this document are hereby incorporated by reference.
The codec in one base station controller can switch to the bypass mode as a result of an in-band hand-shaking operation with the codec in the other base station controller. Transmitting control information from one codec to the other over the audio data stream allows this hand-shaking operation to take place. The control information is transmitted by bit stealing. This is effected by inserting in selected PCM samples bits from the control information signal. Once the handshaking operation is completed, the decoder of the codec in one base station controller and the encoder of the codec in the other base station controller are caused to transition to the inactive mode.
An element of the bypass mode is the importance of ensuring the integrity of the data being transmitted between the two bypass capable devices. While the nature of a PCM speech sample is unlikely to be severely effected by possible bit manipulation that may arise during transmission over either the forward link or the return link of the data communication channel, a small change to the bits of a compressed audio data signal will have important detrimental consequences on the quality of communication over the link. Thus, codec bypass operations rely on a clear data communication channel between the two bypass capable devices. Channel noise or impairments present on the inter-device link may alter the traffic and/or signaling information bit pattern transferred between the two devices and could disrupt bypass operations causing severe degradation to the communication. Inter-device link impairments can be generated by poorly engineered connections, by jitter due to a packet network or by In-Path Equipment (IPE) such as A/D converters, D/A converters, echo cancellers, gain pads or conference bridges. Most of the IPEs alter the signal transferred on the link, completely prohibiting codec bypass operations before proceeding beyond the negotiation stage via PCM in-band signaling. Consequently, the communication remains in non-bypass mode with no noticeable degradation due to the impairment. However, some improperly conditioned connections, jitter or IPEs alter the signal lightly or selectively but could nonetheless induce severe degradation to the vocoder bypassed communication. Codec bypass operations under the influence of such noise and impairments may manage to proceed beyond the negotiation stage such that the communication enters the codec bypass mode. Bypass traffic information or signaling information will then be subject to important distortion by the impairments. In the case of traffic information distortion, end-to-end quality is poor for the duration of the codec-bypassed conversation. In the case of signaling information distortion, bypass stability is perturbed. Overall, the effect on the communication can be severely objectionable and annoying.
A particular problem also arises when the link impairment is of a magnitude such as to cause the bypass capable devices to exit the bypass mode momentarily. The existing bypass logic is such that the bypass capable devices will attempt immediately to re-engage the bypass mode. If they manage to do so under the same link impairment conditions, the bypass mode will be engaged and likely to be lost soon thereafter. This cycling is objectionable, as the quality of the transmission is highly degraded.
Against this background it clearly appears that there is a need in the industry to improve the behavior of signal processors, in particular bypass capable signal processors, in the case where link impairments develop in the data communication channel.
The present invention provides a signal processor for effecting the conversion of an audio data signal from one format to another. The signal processor has a signal converter that can selectively acquire two operative modes, namely a first operative mode and a second operative mode. In the first operative mode, the signal converter transforms the audio data signal from one format to another and releases the converted audio data signal from the output of the signal processor. In the second operative mode, the signal converter is disabled and permits passage of the audio data signal to the output without conversion.
The signal processor has a control unit for controlling the transition of the signal converter between operative modes. The control unit enables the signal converter from passing from the first operative mode to the second operative mode when at least one operating condition has been satisfied. In a specific, non-limiting example of implementation, the operating condition can be the reception from a companion signal processor of a certain number of error-free control messages during a handshaking operation. The present invention also covers the situation where a plurality of operating conditions need to be met to allow the transition from the first operative mode to the second operative mode.
The control unit reacts to a link impairment occurring on the data communication channel to alter an operating condition to be satisfied to enable passage from the first operative mode to the second operative mode. In a specific non-limiting example of implementation, the modification to the operating condition can be such as to xe2x80x9ctightenxe2x80x9d the requirements for switching from the first to the second operative modes. For instance, when the original operating condition is the reception of a number of error-free control messages during the handshaking operation, the altered operating condition requires a higher number of error-free control messages to be received to allow the signal converter to switch subsequently from the first operative mode to the second operative mode. This feature prevents or at least reduces the likelihood of undesirable cycling of the signal converter between its operative modes.
The signal processor according to the invention can find applications in digital communication systems, such as a digital cellular system or a Voice-over-IP (VoIP) system, in particular codec bypass capable systems that can selectively enable the activation or de-activation of the encoding and decoding functions in the connection. In a preferred embodiment, the audio data signal is an encoded signal that includes a succession of data frames. The signal converter has a codec with a decoder, located at a base station of the network that receives the audio data signal from the mobile telephone. In the active mode of the signal converter, the decoder converts the audio data signal into PCM format and sends it to a remote base station over a landline network, such as the PSTN (Public Switched Telephone Network). In the inactive mode, the signal converter passes the encoded audio data, namely the compressed data frames, to the output of the signal processor without decoding the data.
At the remote base station that receives the audio data signal from the first base station, the signal converter has a codec with an encoder. In the active mode of the signal converter, the encoder converts the audio data signal from PCM format to compressed format and sends the encoded data to the corresponding mobile telephone. In the inactive mode, the signal converter passes the encoded audio data received from the first base station to the corresponding mobile telephone without re-encoding the data.
The invention also provides a method for processing an audio data signal. During a first mode of operation, the audio data signal is converted from a first format to a second format, where in the first format the audio data signal is compressed data and in the second format the audio data signal is de-compressed data. During a second mode of operation, conversion of the audio data signal from a first format to a second format is omitted. The processing of the audio data signal can be switched from the first mode to the second mode when at least one operating condition is satisfied. The method comprises altering the condition when a link impairment develops, such that subsequent transition from the first to the second mode of operation will necessitate satisfying a different operating condition.
The invention also extends to a transmission system using the signal processor described above.